Mine that was crazy aggressive was a red sex link and an ancona. I have read that red sex links can be like that. I also have been told by many that rhode island reds don't mix well with really docile breeds like buffs, and ameraucanas. My grampa had rhode islands and they were aggressive.
I had this happen to our hens earlier this spring. We got rid of the two most aggressive and everyone has gotten along great since. I considered rehoming the nice ones who got picked on but i figured the bullies would just start on the others. Besides i dont have a need for chickens that...